num = input()
if len(num) ==4:
  n1 = num[0]
  n2 = num[1]
  n3 = num[2]
  n4 = num[3]
  spis = [int(n1),int(n2),int(n3),int(n4),int(n1+n2),int(n1+n2+n3),int(n1+n2+n3+n4),int(n2+n3),int(n2+n3+n4),int(n3+n4)]
  spis.sort(reverse=1)
  for x in spis:
    f = 1
    for num in range(2, x//2+1):
      if x%num == 0:
        f = 0
        break
    if f == 1:
      print(x)
      break
  if f == 0:
    print(0)
else: print(0)
